Method of coating copper or its alloys with tin



c Patented May 23, 1939 UNITED ST TES PATENT OFFICE warrior) or coa'rm'acorraa on. 11's ALLOYS Wrrn 'rm .No Drawing. Application April 5, 1937,Serial No. 135,081

Claims.

Our invention relates to a method of coating copper or its alloys withtin. More specifically,

it relates to a method of coating by an immersion "1 5 Copper and .itsalloys find wide use in the plumbing industry. Water frequently attackstubes or pipes, giving rise to what is commonly termed green water". Therate of formation of metallic salts that cause green water varies withthe pipes used, the purity 'of the water, the

gases dissolved therein, temperature, and other factors; If water iscontinuously flowing through pipes, the concentrationcf salts either insolution, in colloidal form, or in mechanical suspension is usually notgreat enough to cause noticeable coloring of the water or staining ofplumbing fixtures. If the flow is intermittent, it usually is indomestic buildings, the concentration may become high enough to give anugly color to the wateror to stain the fixtures. Formation of colorstains is frequentlyaccentuated by toilet or laundry soaps which forminsoluble colored copper soaps.

in the prior art, attempts havebeen made to coat the inside of pipes andtubes with metallic tin by a hot sweating process which involves theapplication of molten tin to the inner wallsof said pipes Ior'tubes by amechanical swabbing operation. This method is expensive and results 40in a coating which is neither uniform in thickness nor in surfacecontour. Moreover, such a method is limited torelatively short sectionsof such pipe-or tubing. likewise, it is not practicable for coating theinside of a coil of pipe.

For enhancing the beauty of various copper or copperv alloy articles inthe prior art, recourse has been had to the electrodeposition of tin.While this method is satisfactory for many purposes, it is subject tocertain important limitations. For'example, it is not practicable foruse in the'coating of the inner walls of tubes or pipes of appreciablelength, or distorted shape. Also, it is dimcult to electroplate'satisfactorily large articles, especially whenthey are of complicatedshape and design. Likewise, all coatingsof tin which are produced byelectrodeposition tend to be defective because they do not covercompletely the surface to which they are applied and commonly havepinhole openings at numerous points in the coating. 5

In the past, certain small articles, such as pins, safety pins,thimbles, buttons and the likehave been coated with tin by what is knownas contact-immersion processes. These processes consist of placing thearticles in contact with a metal -l0 such as zinc, while immersed in asolution such as a tin salt. This contact metal sets up a galvanic cell,so that in eiiect the process is one of electrodeposition wherein theelectric current is generated by chemical means within the bath. 1 Theseprocesses have very limited application and are not suitable for coatingthe interiors of pipes or tubes.

In the art of deposition by the contact process, as commonly practiced,the baths utilized normally contain such combinations as a saturatedsolution of cream of tartar containing stannous chloride, a solutioncontaining from 20 to 30 grams of ammonium alum and l to 2 grams ofstannous chloride per liter. or a sodium stannite 25 solution containingabout 7 grams per liter of sodium cyanide. Coatings obtained from suchbaths are ordinarily extremely thin, do not have good covering powersand are comparatively expensive. Furthermore, these coatings can beapplied only to small and relatively simple articles and cannot bapplied to pipes and tubes of appreciable length and particularly to theinterior thereof. Likewise, these baths must be operated usually atelevated temperatures, approaching the boiling point and the tin saltsare unstable under these conditions of operation, with the result thatthe baths rapidly become depleted. Likewise, the time required forcoating with these baths is usually long, frequently amounting to 40several hours.

One of the objects of this invention is to obtain coatings of tin oncopper or its alloys or copper alloy coated articles by a simple andrelatively inexpensive process wherein the coating may be produced in arelatively brief time.

Another object of this invention is to produce such a coating of tin onthe materials specified by the use of a bath at atmospheric temperatureand without the use of an electric current.

Another object of this invention is to produce coatings of tin onmaterials of the type indicated which will be sufilciently thick andwhich will have ample covering power. 55

Other objects will appear from the following description and claims.

Our invention involves the discovery that tin coating can be applied tocopper or its alloys by an immersion process at ordinary temperaturesand without the use of electric current from the outside or any contactmetal'within the baths. The invention is based on the fact that bathscontaining'c ertain ingredients in selected proportions will beeffective to produce a coating of tin upon a copper or copper alloywithout the use of any contact metal or of any electric current from theoutside.

It has been discovered that, if a bath contains stannous tin and asufllciently high concentration of cyanide, coatings of excellentquality can be produced on copper or copper alloys with the bath at roomtemperature. Although the essential characteristics of the bath requirethe presence of a high concentration of cyanide and the presence ofstannous tin, the latter can be present in the solution in variousforms. Likewise, different raw materials can be utilized to prepare thebath. Thus, it is possible to utilize a bath comprising sodium stanniteand sodium cyanide or a bath of stannous tartrate or citrate and sodiumcyanide, However, these baths are illustrative only and, although theydo not exhaust the possible sources of stannous tin, they show the widerange of materials that can be used.

It is obvious, of course, that sodium stannite may be prepared by addingto the bath required amounts of stannous salts such as stannous chlorideand sodium hydroxide. Likewise, tin tartrate can be made from tartaricacid and a suitable tin salt. Potassium, ammonium'or other alkali saltscan be used instead of sodium salts.

One form of our method will be illustrated by considering a sodiumstannite-sodium cyanide bath. We have prepared baths of sodium stanniteby reaction of stannous chloride and sodium hydroxide according to thefollowing reaction: SnClz+4NaOH=NazSnOz+2NaCl A sodium stannite bath soprepared is ready for use on addition of sodium cyanide. Theconcentration of sodium stannite can vary from gram per liter to asolution substantially saturated. In fact, the concentration of sodiumstannite may be lower than gram per liter but, if so, the'solutionbecomes depleted quite rapidly on use. The concentration of cyanide mustbe high, but the lower limit that can be used varies with the timeallotted for coating. The following example will illustrate theconcentration of sodium cyanide that can be used under varying conditions: A bath was prepared containing 5 grams per liter SnClz.2HzO and5.6 grams per liter NaOH, The cyanide concentration was varied. Asolution containing 5 grams per liter NaCN at room temperature gave butlittle visible coating in less than one hour, and the coating secured inthat time was thinand incomplete. The'lsurface of the copper was notprotected. When the concentration of NaCN was 15 grams per liter, avisible coating was obtained in about four to five minutes, butimmersion for approximately an hour was required to give complete andadequate coverage. At 25 grams NaCN per liter, a visible coating wasobtained in two minutes, and satisfactory coverage was'obtained in tenminutes or less. At 50 grams and above per liter, visible coatingswereobtained in one minute or less. We have made coatings with solutionscontaining as much as 200 grams NaCN per liter. The

example above is illustrative only and our invention is not limited tothe concentration of SnCl: or NaOH given in the example.

In the operation of our process, the copper or copper alloy article ispreferably properly cleaned or cleaned and pickled in the mannerfollowed for preparing such articles for plating, coating or the like,although in some instances such cleaning or cleaning and picklingoperations are not absolutely necessary. Ini'act, the solution itselfhas cleansing properties. The article is immersed in thestannite-cyanide bath. Immediately upon immersion, tin commences todeposit on the article in plate-like form. In many cases, a satisfactorycoating is obtained in one minute or less, although in some cases alonger period is required. In general, the longer the period ofimmersion, the heavier will be the coat obtained. Although we do notcare to be bound by any particular theory, we believe that the coatingresults from an electrochemical replacement, with the copper going intosolution and the tin plating 'out. The bath may be operated at roomtemperaure.

If it is desired to coat the inside of pipes, tubes portantcharacteristic is high cyanide content of the bath. Tin tartrateimmersion plating baths may be made by using varying amounts of sodiumstannite, tartaric acid, and sodium cyanide. stannous chloride may besubstituted for sodium stannite. In the preparation of a bath fromstannous chloride, tartaric acid, and sodium cyanide, the concentrationof stannous chloride can be varied over wide limits 'but,in order to getsatisfactory performance, the concentration of tartaric acid and cyanidemust be controlled more carefully, For example, with a bath containing 5grams per liter SnClz.2HzO and 2Q grams per liter tartaric acid, theaddition of NaCN causes a precipitation of part of the tin fromsolution. However, such a bath will yield coatings, provided theconcentration of NaCN exceeds 35 grams per liter. However, increasingthe concentration to 50 grams or more NaCN per liter eflected much morerapid coating. Addition of suflicient tar-' taric acid permits making asolution from which tion was alkaline, and. a heavy sludge formedr" With10 grams per liter, the solution was still alkaline and a heavy sludgealso formed. With 30 grams per liter, the sludge was less but thesolution was still alkaline: With 50 and '75 gramsper liter, the sludgeswere very slight and the; When 100 grams per' solutions were alkaline.liter was used, the sludge was slight but the solution was acidic. Thesolutions deposited tin on copper by immersion so long as the solutionswere alkaline in nature. The prerequisites of a tartaric-cyanide bath,-therefore, are suilicient tartaric acid or soluble tartrate to preventprecipitation of tin on addition of NaCN, and sufllcient NaCN tomaintain the solution alkaline in nature. An excess of cyanide over thatrequired to make apply a tin coating to-the thus-prepared surface by themethod of this invention.

It has also been discovered in some cases that superiorv coatings can beobtained by applying a coating in the manner described above. and thenheating the tin-coated material for a period at a low temperature, andthen applying a second coating by again immersing the article in thecoating solution. The procedure may be repeated several times, ifdesirable. We have found that by heating at about 230 C. for a timesufllcient to bring the article to temperature or longer gives thedesired results. In some cases, it is desirable to heat substantiallyabove 230C. to effect alloying of the tin with the base metal.

' As pointed out, our invention in its preferred form contemplatesapplying, a coating of tin to copper or its alloys by immersion in asolution containing stannous tin'and appreciable amounts of cyanide. Asalso pointed out, the bath may be operated at room temperatures but theinvention is not limited thereto. We may, however, for example, carry onthe coating operation at temperatures up to the boiling point of thesolution used. I

This invention is particularly advantageous because it yields coatingsthat are continuous and with a highly metallic sheen. In fact, coatingsare equal to and, in many cases, superior in appearance to thoseobtained by electroplating. The bath can be operated at low temperaturesand does not require the expensive method of heating necessary in priorart contact-immersion baths. It is more stable than prior artcontactimmersion baths. Furthermore, the necessity of using a contactmetal is eliminated, thus simplifying the operation of the bath anddecreasing the cost of operation. The quality of coating, especially asregards continuityand freedom from pinholes, exceeds that of coatingsfrom prior art contact-immersion baths. Coatings are obtained by asimple operation. This is decidedly advantageous since by this methodthe inside of coils of pipe can be coated, whereas such an operationcannot be effectively performed by any prior art operation, includingswabbing with molten tin, electroplating, or immersion by the contactprocess. It is also possible by the process of this invention to coatthe inside of pipe, only, thus eifecting a material saving in the amountof tin used.

It has been found that trouble due to green water in plumbing fixturesmay be eliminated by application of a coating of tin to the inside ofpipes or tubes by the method of this invention. It has also beenpossible by use of this invention to coat various copper and brassarticles to give a bright metallic tin coating that is markedlyresistant to atmospheric corrosion. In short, this invention hasresulted in the production of a protective or decorative coating onarticles of copper or its alloys or on copper or copper alloy coatedarticles at a remarkably low cost and in a simple manner.

It is within the scope of our invention to use, instead of sodiumcyanide or hydroxide, other soluble cyanides or hydroxides, such assodium, potassium, ammonium. Likewise, instead of stannous chloride,other stannous salts may be used, for example, the sulphate.

Having thus described our invention, what we claim is:

l. The method of forming an adherent immersion coating of tin on thesurface of copper or its alloys, said method consisting essentially incontacting said surface with an alkaline aqueous solution containingdissolved tin in the stannous condition and a soluble cyanide, thecyanide being in a concentration greater than '1 grams per liter.

2. The method of claim 1, wherein the said solution also contains asubstance selected from the group consisting of tartaric acid,tartrates, citric acid, and citrates.

3. The method of claim 1, wherein the tincoated surface produced isheated to a temperature of at least 230 C. and the coating process isrepeated.

4. The method of claim 1, wherein the tin is present in the solution assodium stannite.

5. The method of claim 1, wherein the said cyanide is sodium cyanide.

- ARNOLD E. PAVLISH.

JOHN D. SULLIVAN.

